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feature articles
IUCrJ (2015). 2, 85–94 doi:10.1107/S205225251402329X 85
IUCrJISSN 2052-2525
PHYSICSjFELS
Received 31 July 2014
Accepted 22 October 2014
Edited by X. Zhang, Tsinghua University, China
†
Keywords: Li-ion batteries; cathode materials;
transition metal fluoride phosphates; structure–
property relationships
Perspectives on Li and transition metal fluoridephosphates as cathode materials for a newgeneration of Li-ion batteries
Evgeny V. Antipov,* Nellie R. Khasanova and Stanislav S. Fedotov
Department of Chemistry, Lomonosov Moscow State University, Moscow 119991, Russian Federation. *Correspondence
e-mail: [email protected]
To satisfy the needs of rapidly growing applications, Li-ion batteries require
further significant improvements of their key properties: specific energy and
power, cyclability, safety and costs. The first generation of cathode materials for
Li-ion batteries based on mixed oxides with either spinel or rock-salt derivatives
has already been widely commercialized, but the potential to improve the
performance of these materials further is almost exhausted. Li and transition
metal inorganic compounds containing different polyanions are now considered
as the most promising cathode materials for the next generation of Li-ion
batteries. Further advances in cathode materials are considered to lie in
combining different anions [such as (XO4)n� and F�] in the anion sublattice,
which is expected to enhance the specific energy and power of these materials.
This review focuses on recent advances related to the new class of cathode
materials for Li-ion batteries containing phosphate and fluoride anions. Special
attention is given to their crystal structures and the relationships between
structure and properties, which are important for their possible practical
applications.
1. Introduction
Today, 90% of the energy used in the world comes from fossil
fuels, causing their rapid consumption followed by ecological
damage and climate change. Together with the demand to
increase the efficiency of using fossil fuels, there is a strong
impetus to develop clean energy technologies, where new
energy-storage devices will play an important role. Energy-
storage devices allow energy buffering for peak use when
power generation systems are operated at peak capacity. They
are critical elements for solar, wind and water renewable
energy technologies, mobile information and communications
technologies, and hybrid/electric vehicles. In all these appli-
cations, electrochemical energy-storage devices have signifi-
cant advantages over other competing technologies because
they can offer much higher energy densities.
Li-ion batteries (LIBs) were originally developed for
portable electronic devices, but nowadays new application
niches are envisaged in electric vehicles and stationary energy
storage. The rapid development of LIB production in the
world during the last two decades was driven mainly by their
gravimetric energy density (now exceeding 250 W h kg�1),
which is several times higher than for alternative rechargeable
battery systems. LIB systems also provide significant advan-
tages in cyclability (the number of operating charge/discharge
cycles), energy efficiency, absence of ‘memory’ effects, low
discharge rate etc. The main disadvantages are relatively high
costs and safety issues. To satisfy the needs of rapidly growing
applications, Li-ion batteries require further significant
improvements to their key properties: specific energy and
power, cyclability, safety and costs.
The gravimetric energy density of an LIB is the product of
its specific capacity (measured as the reversible charge trans-
ferred between the anode and cathode per unit weight) and
the potential difference between these electrodes where
reversible intercalation/deintercalation of Li ions occurs. The
cathode is a key part of LIBs that significantly determines
their performance. Severe requirements are imposed on the
cathode material, which should demonstrate a high specific
capacity, provide fast reversible intercalation of the Li ions at
a redox potential close to the upper limit of the electrolyte
stability window, possess a high electronic conductivity and a
relatively low molecular weight, and exhibit a small variation
in volume upon changing the Li concentration over a wide
range. These properties are covered mainly by the crystal
structure of the cathode material, and this is the reason for the
large impact of crystallography research on the design of new
cathode materials for LIBs and the optimization of their
properties by appropriate modification of the chemical
composition and, consequently, the crystal structure. It should
be emphasized that the requirement for a high specific capa-
city (Ct) of the cathode material restricts the number
of elements that can be used due to the following
equation:
Ct ¼26:8n
MðAh g�1
Þ; ð1Þ
where n is the number of Li ions (and electrons) participating
in the reversible redox reaction and M is the molecular weight
of the material. By this equation, the selection of elements in
the chemical formulae of Li-based compounds is limited to
light transition metals such as Ti, V, Mn, Fe, Co and Ni. The
requirement for fast Li-ion diffusion in the crystal structure of
the cathode material (as it is a rate-limiting step of the redox
process) is the key to achieving high-power LIBs because the
diffusion time in a crystallite of the material is proportional to
R2/D, where R is the crystallite size and D is the diffusion
coefficient.
Goodenough and co-workers explained, in a paper
published in 1980 (Mizushima et al., 1980), the selection of
LiCoO2 as a possible cathode material with high specific
energy because it possesses a suitable type of crystal structure.
This �-NaFeO2-type structure is a layered derivative of the
rock-salt structure, with a complete ordering of the Li and Co
cations in the octahedral cavities between alternating close-
packed oxygen layers. It is relatively light-weight, has a high
average potential for Li-ion extraction (and Co3+/Co4+ tran-
sition) (close to 4 V versus Li+/Li0) and contains empty
tetrahedral voids linked by common faces to the LiO6 octa-
hedra that provide fast Li-ion two-dimensional diffusion,
making this material attractive for LIBs. Ten years later, this
discovery gave rise to the LIB industry, which nowadays
requires further improvement in the cathode material prop-
erties for lower-cost longer-life higher energy/power density
batteries, thus resulting in active development and research in
this field. Since Goodenough’s pioneering work, various
classes of cathode materials have been discovered, such as
compounds based on the NASICON-like crystal structure
[Li3Fe2(PO4)3], spinel LiMn2O4 with a three-dimensional
pathway for Li-ion diffusion, and olivine LiFePO4 with a one-
dimensional pathway, to name just a few.
The investigation of the NASICON-like compounds
Li3+xFe2(XO4)3 (X = P, S, Mo and W) performed by Good-
enough and co-workers in the late 1980s allowed the formu-
lation of the inductive effect concept, which explains the
variation in electrode potential for these materials with the
same structure type and close Fe—O bond distances by the
changing electronegativity of the tetrahedrally coordinated X
cation. This finding was an important step in postulating the
‘structure–property’ relationship for these cathode materials
that enabled further tuning of their properties. A variation in
the electronegativity of the X cation causes weakening or
strengthening of the covalency of the Fe—O bonds, thus
changing the energy of the Fe3+/Fe2+ redox couple and the
respective electrode potential. For these materials, the elec-
trode potential increases from 2.8 to 3.6 V versus Li+/Li0 from
the less electronegative P cation to the most electronegative S
cation, due to the higher ionicity of the Fe—O bonds
for the sulfate-based compound (Goodenough & Kim,
2010).
The first generation of cathode materials for LIBs, based
on mixed oxides with either spinel (LiM2O4, M = transition
metal) or rock-salt derivatives (LiMO2), has already been
widely commercialized, but the potential to improve the
performance of these materials further is almost exhausted.
Li and transition metal inorganic compounds containing
different polyanions (XmOn)p� (X = B, P, S, Si) are now
considered as the most promising cathode materials for
the next generation of LIBs for electric vehicles and large
storage applications because their frameworks can provide
long-term structural stability, which is essential for cyclability
and safety. The best example among these materials is
LiFePO4, discovered in 1997, which has already been widely
commercialized (Padhi, 1997). Further advances in cathode
materials are anticipated to be achieved by combining
different anions [such as (XO4)n� and F�] in the anion
sublattice, which is expected to enhance the specific energy
and power of these materials. Various fluoride phosphates
(often called fluorophosphates) and fluoride sulfates have
recently been discovered, and some of them do indeed exhibit
attractive electrochemical performance. A wide selection of
various polyanion-based cathode materials is described
in the comprehensive review by Masquelier & Croguennec
(2013).
In this review, we will focus on recent advances
in the new and relatively narrow class of cathode
materials for LIBs containing phosphate and fluoride
anions. Special attention will be paid to their crystal
structures and the relationships between structure and
properties, which are important for their possible
practical applications.
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86 Evgeny V. Antipov et al. � New materials for Li-ion batteries IUCrJ (2015). 2, 85–94
2. LiFePO4: crystal structure and Li-ion diffusionpathway
Earth-abundant, environmentally benign and safe lithium iron
phosphate, LiFePO4, has now reached prominence as a
commercialized cathode material in LIBs for hybrid vehicles
and grid storage systems. Upon Li extraction this material
exhibits a flat plateau around 3.4 V versus Li+/Li0, because of a
two-phase redox reaction mechanism involving the formation
of fully delithiated FePO4.
The crystal structure of olivine-type LiFePO4 can be
described as a slightly distorted hexagonal close-packed
(h.c.p.) oxygen array with an orthorhombic unit cell (space
group Pnma). Within the h.c.p. oxygen framework, the Li and
Fe cations are located on half of the octahedral sites and the P
atoms are located on 1/8 of the tetrahedral sites. The corner-
sharing FeO6 octahedra generate layers (perpendicular to the
a axis) that are bridged by the corner- and edge-sharing PO4
tetrahedra to form the structure framework with strong three-
dimensional bonding. The LiO6 octahedra share edges and
create linear chains running along the b axis.
It is fascinating how state-of-the-art technology has
converted an insulator-type compound with poor electronic
conductivity into a high-rate charge–discharge nanostructured
electrode material, which delivers a specific capacity close to
its theoretical value (170 mA h g�1) and exhibits a relatively
high energy density of 583 W h kg�1. To achieve this electro-
chemical performance, many studies of the kinetics of the
material and its intrinsic Li-ion mobility have been performed,
including those using modern computational and first-princi-
ples methods such as density functional theory (DFT) and
molecular dynamics (MD).
In 2005, Islam and co-workers
demonstrated for the first time an
application of the MD approach to
the examination of Li+-ion diffu-
sion pathways in LiFePO4 (Islam et
al., 2005). Despite the presence of
two crystallographically possible
channels for ion migration in the
structure (along the b and c axes),
the calculated activation barriers
for Li-ion hopping confirmed the
preferential accessibility of only the
[010] direction, which proved the
concept of one-dimensional trans-
port for this material (Fig. 1a). It
was shown that Li+ ions follow a
curved pathway, jumping through
adjacent tetrahedral and octahe-
dral voids, with an energy barrier of
0.55 eV. These results are in good
agreement with the DFT calcula-
tions performed by Morgan et al.
(2004).
At that time, the study of Li-ion
transport was limited to computa-
tional predictions, and no experi-
mental evidence of the diffusion
pattern had been obtained until
Nishimura et al. (2008) clearly
visualized the migration pathway
by means of high-temperature
powder neutron diffraction and the
maximum entropy method (MEM)
(Fig. 1b). The refined anisotropic
atomic displacement parameters
for Li ions at room temperature
provided significant information on
the overall anisotropy of the
thermal vibration according to the
shape of the ellipsoid. The thermal
displacement is preferentially
feature articles
IUCrJ (2015). 2, 85–94 Evgeny V. Antipov et al. � New materials for Li-ion batteries 87
Figure 1Visualizations of the curved one-dimensional Li-ion diffusion pathway in the LiFePO4 crystal structureby various methods. (a) From Islam’s work based on the MD approach. Adapted with permission fromIslam et al. (2005). Copyright (2005) American Chemical Society. (b) The nuclear distribution of Li atomsgiven in the [010] and [001] directions, respectively. Adapted by permission from Nishimura et al. (2008).Copyright (2008) Macmillan Publishers Ltd, Nature Materials. (c) LiFePO4 BVS map sliced along the abplane. (d) The crystal structure of LiFePO4 with an incorporated BVS map. Li ions are depicted as orangespheres, and blue octahedra and green tetrahedra designate FeO6 and PO4 polyhedra, respectively.
oriented towards adjacent face-sharing vacant tetrahedral
voids, which produces the expected wave-like one-dimen-
sional chain of Li+ ions.
The next experimental stage was to show the conversion of
Li+-ion vibrations to actual diffusion. With this aim, the
Li0.6FePO4 solid-solution composition was obtained at
elevated temperature (above 570 K) with a significant number
of lithium defects. Conventional structure models that include
harmonic vibration of a static atom could not be exploited to
investigate the dynamic disorder of the Li ions in the partially
delithiated phase. The application of the MEM allows one to
describe the complicated electron density or nuclear distri-
butions that are beyond the classical static structure model,
and therefore to evaluate the dynamic disorder of the Li ions
by estimating the neutron scattering length density distribu-
tion. In crystallography, the MEM approach was first intro-
duced by Collins (1982) and then validated for various
materials with high ionic conductivity (Yashima et al., 2003,
2005).
The resulting contour maps of the nuclear distribution of Li
atoms sliced along the [010] and [001] directions are given in
Fig. 1(b). The curved one-dimensional pathway along the [010]
direction was successfully observed, providing long-expected
experimental proof of the Li diffusion pathway in LixFePO4.
These findings are also consistent with the computational
examinations given by Islam et al. (2005) and Morgan et al.
(2004).
Both the above-mentioned theoretical and experimental
approaches for probing Li-ion diffusion in similar materials
are time-consuming, difficult to execute and sometimes
require expensive preparations. In this case, a scientist should
carefully choose the objects for these tests. That is why Li-ion
migration is thoroughly examined by these approaches mainly
in ‘renowned’ materials such as LiFePO4. However, there are
many alternative electrode materials that are of interest for
their Li-ion transport maps or overall kinetics. This fact
facilitates the development of quick and simple tools for the
preliminary investigation or prediction of ion-diffusion path-
ways in a material’s crystal structure.
Bond valence (BV) calculations were at first recognized as a
rapid and efficient approach for validating the crystal struc-
tures of inorganic compounds (Brown, 2009). Later on, the
method found some new applications. In particular, spatial
distributions of BV values have been demonstrated to yield
information concerning the topology of ion-conduction path-
ways (Adams, 2012; Adams & Rao, 2014). With this method to
hand, one can not only draw tentative conclusions on the
character of ion diffusion, but also significantly reduce the
time and costs required for further examination by computa-
tional or complicated experimental techniques.
The BV sum (BVS) landscape for LiFePO4 perfectly
reproduces the curved-shaped distribution of Li ions along the
[010] direction. In Figs. 1(c) and 1(d), one may observe a
zigzag pathway running along the b axis and passing through
neighbouring occupied octahedral 4a sites and vacant tetra-
hedral sites. The intermediate tetrahedral positions for Li-ion
hopping show very low BVS mismatch values. In other words,
these sites are demonstrated to have a reasonably high
probability of participating in Li-ion diffusion. To sum up, the
BVS data accurately correlate with computational and
experimental methods, which gives us an opportunity to use
BVS calculations for preliminary prediction and character-
ization of the Li migration pathways in the cathode materials
for LIBs that are discussed below.
In this paper we present BVS calculations for a set of
cathode materials for LIBs. The calculations were performed
using the 3DBVSMAPPER program (Sale & Avdeev, 2012).
The set of materials comprises LiFePO4, LiVPO4F with the
tavorite structure type, and fluoride phosphates with the
general formula Li2MPO4F (M = Mn, Fe, Co), adopting three
different types of framework.
3. Li and transition metal fluoride phosphates: crystalstructure and Li-ion diffusion pathway
Further advances in polyanion cathode materials are related
to joining different units in the anion sublattice, which gives
opportunities for varying the chemical composition and
structural framework of the compounds and to tailor their
electrochemical properties. Fluorine and oxygen, which are
similar in size, can easily substitute each other in the anion
sublattice. This combination of (XOn)p� and F� anions is
expected to increase the operating voltage due to the higher
ionicity of the M—F bond, which would ultimately provide a
higher specific energy density. The difference in the formal
charges of O2� and F� is expected to weaken Li bonding to the
structure framework and enhance Li-ion transport, thus
improving the power-density parameters. Bearing these
considerations in mind, chemists have attempted to synthesize
various fluoride-based polyanion cathode materials.
3.1. Fluoride phosphates with the tavorite structure type
The first fluoride-based polyanion cathode material was
introduced by Barker et al. (2003), who reported Li-ion
insertion/extraction activity in the fluoride phosphate phase
LiVPO4F, prepared by the carbothermal reduction method.
LiVPO4F is isostructural with the mineral tavorite,
LiFePO4OH, and crystallizes in a triclinic unit cell (space
group P1, Z = 2) with a = 5.1688 (2) A, b = 5.3094 (2) A, c =
7.4994 (2) A, � = 113.12 (1)�, � = 112.93 (2)�, � = 81.66 (2)�
and V = 174.3 A3. Its crystal structure is built up of two slightly
distorted crystallographically independent (VO4F2) octahedra
bridged by fluorine vertices in the trans-positions, to form one-
dimensional chains along the c direction. These (VO4F2)1chains are interconnected through phosphate groups and this
packing creates a three-dimensional framework with wide
tunnels that host the Li ions (Fig. 2a). According to Ellis et al.
(2011), the Li ions are randomly distributed over two crys-
tallographic positions separated by 0.79 A: the five-coordi-
nated Li1 and six-coordinated Li2 sites. The occupancies of
the Li1 and Li2 sites were found to be 0.18 and 0.82, respec-
tively, and this finding was confirmed by NMR measurements.
From combined X-ray and neutron powder diffraction data,
feature articles
88 Evgeny V. Antipov et al. � New materials for Li-ion batteries IUCrJ (2015). 2, 85–94
Ateba Mba et al. (2012) suggested an occupancy for the single
Li site corresponding to the major occupied position reported
by Ellis and co-workers.
The structural framework of LiVPO4F implies four crys-
tallographically distinct channels (along the [100], [010], [101]
and [111] directions) suitable for Li-ion migration. BVS
calculations for this material revealed a very complicated
diffusion pathway for the Li ions. As shown in Fig. 2(b), it
passes preferentially along the [111] direction. This finding is
in good agreement with the work of Mueller et al. (2011)
claiming that this direction, with the lowest energy barrier of
208 meV, is the most favorable for Li-ion migration.
The electrochemical activity of LiVPO4F is a good example
representing the inductive effect caused by fluorine. The
potential of Li deintercalation involving the V4+/V3+ redox
couple is near 4.2 V, i.e. about 0.5 V higher than the potential
for the same transition observed in Li3V2(PO4)3 with the
NASICON structure (Gaubicher et al., 2000), whereas the
observed capacity value of 155 mA h g�1 corresponds to the
reversible uptake of 0.97 Li ions (Barker et al., 2003). More-
over, this system exhibits the possibility of realising multi-
electron redox activity based on the multivalent nature of
vanadium: the reversible insertion of 0.9 Li ions (involving the
V4+/V3+ redox transition) takes place at �1.8 V and corre-
sponds to a capacity value of 140 mA h g�1 (Barker et al.,
2005). Both redox reactions were shown to be covered by a
two-phase mechanism and accompanied by significant volume
changes: about 8.5% for the deintercalation reaction
(LiVPO4F ! VPO4F) and 7.4% for the LiVPO4F !
Li2VPO4F transition (Ellis et al., 2011). Although the total
capacity value of Li1+xVPO4F exceeds 290 mA h g�1, the large
difference between these two redox reactions (2.4 V) makes
the application of this system in its multivalent form
problematic. An application of this material in a symmetrical
cell at both positive and negative electrodes was proposed by
Barker (2005), but it exhibited poor cycling stability due to
dissolution of LiVPO4F at the anodic site in the acidic LiPF6-
based organic electrolyte. Nevertheless, switching to an ionic
liquid electrolyte provided a much more stable and highly
reversible performance of the symmetrical cell at room and
elevated temperatures (Plashnitsa et al., 2011).
Despite the attractive electrochemical properties demon-
strated by the Li1+xVPO4F system, the high cost and toxicity of
vanadium compounds inspired a search for new fluoride
phosphate cathode materials among other 3d transition
metals. Lithium iron fluoride phosphate, LiFePO4F, iso-
structural with tavorite, was prepared by different synthetic
routes, including solid-state reaction and ionothermal or
solvothermal synthesis (Recham et al., 2010; Ramesh et al.,
2010; Ellis et al., 2012). It crystallizes in the triclinic space
group P1 with cell parameters a = 5.1516 (2) A, b =
5.3002 (2) A, c = 7.2601 (2) A, � = 107.343 (3)�, � =
107.880 (3)�, � = 98.559 (3)� and V = 173.67 (6) A3. According
to Rietveld refinement of powder X-ray diffraction data, the
Li ions are located on two independent crystallographic sites,
Li1 and Li2, with partial occupancies of 0.75 and 0.25,
respectively (Ellis et al., 2012).
In contrast with LiVPO4F, lithium extraction from
LiFePO4F was not observed since it would involve the Fe3+/
Fe4+ redox reaction taking place at very high potentials.
LiFePO4F is capable of reversible Li intercalation, with an
average redox potential of 2.8 V and a reversible capacity of
145 mA h g�1, close to the theoretical value (146 mA h g�1).
Li insertion results in considerable volume expansion (�8%)
and gives the new Li2FePO4F phase. Its structure was refined
in the space group P1 [a = 5.3276 (2) A, b = 5.3736 (2) A, c =
7.4791 (2) A, � = 108.271 (4)�, � = 108.398 (4)�, � =
94.615 (4)�) and V = 189.03 (4) A3] on the basis of combined
X-ray and neutron powder diffraction data. It was found that
Li insertion keeps intact the corner-shared ‘FePO4F’ frame-
work of the parent tavorite structure and leads to the
appearance of two additional sites for the Li ions. According
to electrochemical and X-ray diffraction data, Li intercalation
in LiFePO4F is rather complex, as it involves occupation of the
first Li site via a solid-solution (single-phase) process, followed
by occupation of the second Li site through a two-phase
mechanism (Ellis et al., 2012). LiFePO4F exhibits excellent
electrochemical performance with low polarization and good
cycling retention (Ramesh et al., 2010; Ellis et al., 2012).
However, the low redox voltage makes its applications ques-
tionable, due to its low energy density, and limited to batteries
with a metallic Li anode (Li metal batteries).
3.2. Fluoride phosphates with general formula A2MPO4F
These compounds with A = Li, Na and M = Fe, Mn, Co and
Ni have received particular interest due to their potential to
operate on more than one alkali atom per transition metal,
which would result in high specific capacities
(>200 mA h g�1). Depending on the nature of the alkali
metals, the 3d transition metals and the synthesis conditions,
these fluoride phosphates adopt three structure types. In all
three types, transition metals occupy the octahedral sites, but
the connectivity of the MO4F2 octahedra is different, varying
from mixed face-shared and corner-shared in the layered
structure [(Na,Li)2MPO4F with M = Fe, Co, Ni; Ellis et al.,
2010] to edge-shared in the three-dimensional orthorhombic
structure (Li2MPO4F, M = Co, Ni; Dutreilh et al., 1999; Okada
feature articles
IUCrJ (2015). 2, 85–94 Evgeny V. Antipov et al. � New materials for Li-ion batteries 89
Figure 2Representations of the LiVPO4F phase. (a) The crystal structure. (b) ABVS map of the Li+-ion transport pathway. Transition metal octahedraare shown in blue, phosphate tetrahedra in green, F in grey and Li ions inorange.
et al., 2005) and corner-shared in the three-dimensional
monoclinic structure (Na2MnPO4F; Yakubovich et al., 1997).
3.2.1. The crystal structures of Na2MPO4F (M = Fe, Co,Ni). They consist of a layered framework described in the
orthorhombic space group Pbcn. Bi-octahedral M2O7F2 units
comprising face-sharing MO4F2 octahedra are connected via
bridging F atoms to form chains, and these are interconnected
by PO4 tetrahedra to yield [MPO4F] infinite slabs. The Na
atoms occupy two distinct crystallographic sites located in the
interlayer space and possess facile two-dimensional migration
pathways (Fig. 3a). Although this type of fluoride phosphate
has been stabilized for different transition metals (M = Fe, Co,
Ni), detailed investigation of their structures and electro-
chemical properties was only carried out for the iron-based
fluoride phosphate Na2FePO4F [a = 5.2200 (2) A, b =
13.8540 (6) A, c = 11.7792 (5) A and V = 851.85 A3; Ellis et al.,
2007].
Na2FePO4F and corresponding electrode materials were
obtained by various synthetic routes (solid-state synthesis, sol–
gel technique, and hydrothermal and ionothermal methods)
which enable the preparation of materials with different
morphologies and particle sizes (Ellis et al., 2007; Recham et
al., 2010). Electrochemical testing of Na2FePO4F in an Li cell
revealed reversible electrochemical activity with an average
potential of 3.3 V (versus Li+/Li0). It delivered a specific
capacity of 115 mA h g�1 (85% of the theoretical value) and
showed good cycling sustainability. These observations
confirmed the possibility of sodium-containing materials
cycling reversibly in lithium-based electrolytes: as Na+ ions
have been extracted from the framework upon electro-
chemical oxidation, Li+ ions become intercalated into
NaFePO4F, resulting in a compositional transformation to
(Na,Li)2FePO4F, which continues to function as an Li cathode
upon subsequent cycling. This new material exhibited a
sloping voltage profile, suggesting a quasi-solid solution
electrochemical behaviour, with a volume contraction of
3.7%, which is much smaller than that observed for LiFePO4.
The electrochemical activity of both Na2FePO4F and
(Na,Li)2FePO4F involves only one electron redox transition,
attributed to the Fe2+!Fe3+ reaction, with a reversible
extraction of one alkali ion. By an ion-exchange reaction, Na
was completely substituted by Li to result in a new Li2FePO4F
phase with a layered two-dimensional structure (a = 5.0550 A,
b = 13.5610 A, c = 11.05200 A, V = 757.62 A3; Ellis et al., 2007).
BVS calculations (Fig. 3b) confirmed the two-dimensional
migration pathways realised in this framework. Therefore, in
the Na2FePO4F structure, both alkali metal sites are open for
facile ion transport, and the limitations observed upon elec-
trochemical deintercalation seem to be caused by the high
potential of the Fe3+!Fe4+ oxidation. Li2FePO4F showed a
voltage charge–discharge profile very similar to that of
Na2FePO4F after five cycles and delivered about 110 mA h g�1
capacity at an average potential of 3.3 V (Ellis et al., 2007).
3.2.2. Fluoride phosphate Na2MnPO4F. This compound
crystallizes in a three-dimensional monoclinic structure [space
group P21/n, with a = 13.683 (3) A, b = 5.317 (1) A, c =
13.711 (3) A, � = 119.67 (3)� and V = 867.1 A3; Yakubovich et
al., 1997]. In this structure, the alternate MnO4F2 octahedra
are corner-shared and connected through fluorine to form
Mn2O8F2 chains, with the F-ion backbone running along the b
axis. These chains are linked by PO4 tetrahedra to form a
three-dimensional framework, with Na+ ions located in the
channels on four independent crystallographic sites (Fig. 4).
Despite an open pathway for alkali-ion migration, initial
reports on Na2MnPO4F revealed the absence of any reversible
electrochemical activity, which was explained by poor Na-ion
diffusion kinetics (Recham et al., 2009; Ellis et al., 2010). Later
on, Wu et al. (2011) prepared a carbon-coated nano-sized
Na2MnPO4F material by a sol–gel technique and tested it in an
Li cell. At elevated temperature (330 K) this material deliv-
ered a reversible capacity of 98 mA h g�1 (for the first
discharge), which faded rapidly upon cycling. Kim et al. (2012)
investigated the electrochemical properties of Na2MnPO4F
prepared through a solid-state reaction and evaluated the
diffusion kinetics of alkali ions using first-principles calcula-
tions. They showed that the b-direction diffusion of Na+ ions
(along the F-ion backbone) is valid in Na2MnPO4F, in agree-
ment with the reversible electrochemical performance of
Na2MnPO4F in an Na cell: the observed discharge capacity of
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90 Evgeny V. Antipov et al. � New materials for Li-ion batteries IUCrJ (2015). 2, 85–94
Figure 3Representations of the Na2MPO4F fluoride phosphate. (a) The crystalstructure. (b) Projections of BVS maps of alkali-ion migration pathwaysin the (011) and (101) layers. The MO4F2 octahedra are depicted in blue,phosphate tetrahedra in green, F in grey, and alkali ions in purple andorange.
120 mA h g�1 indicated the transfer of almost one Na ion.
Furthermore, by an ion-exchange reaction they obtained a
new Li2MnPO4F phase (isostructural with the parent phase),
which demonstrated a discharge capacity of 140 mA h g�1
with a redox potential plateau at around 3.9 V (Kim et al.,
2012).
Analysis of Li-ion transport in the new Li2MnPO4F phase
revealed that, in addition to b-direction diffusion, the Li ions
are able to migrate through pathways perpendicular to the
F-ion backbone, which were forbidden for Na-ion diffusion
because of the high activation energy (Kim et al., 2012). These
results are in a good agreement with the data of BVS mapping
for Li2MnPO4F that clearly indicated two-dimensional
migration pathways: along the b axis and along the diagonal of
the ac plane (Fig. 5). These findings suggested that
Li2MnPO4F would exhibit a more facile Li-ion diffusion.
For both of the manganese-based fluoride phosphates,
Na2MnPO4F and Li2MnPO4F, electrochemical activity was
limited to reversible transfer of about one alkali metal. Kim et
al. (2012) calculated the average voltages for the first and
second alkali metal extractions and showed that, with cut-off
voltages above 4.8 V, more than one Na or Li ion would be
extracted, but the electrolyte used could not sustain these high
voltages.
3.2.3. Fluoride phosphates, Li2MPO4F (M = Ni, Co). These
compounds, with an orthorhombic three-dimensional struc-
ture, can be prepared by direct synthesis. Li2NiPO4F was
reported in 1999 (Dutreilh et al., 1999), while Li2CoPO4F
(space group Pnma, with a = 10.444 A, b = 6.381 A, c =
10.864 A and V = 724.1 A3) was introduced in 2005 by Okada,
who proposed to use it as a high-voltage cathode material
(Okada et al., 2005). The structure type is built of MO4F2
octahedra linked through their edges to form rutile-like
chains, with F atoms in trans-positions (Fig. 6). These parallel
chains are interconnected through phosphate groups, and this
packing creates a three-dimensional framework with large
tunnels along the [010] direction which accommodate the Li
ions in three distinct positions with full occupancy: two Li sites
(Li1 and Li2) are five-coordinated, while the third one (Li3)
has a distorted six-coordinated environment (Dutreilh et al.,
1999; Hadermann et al., 2011). Analysis of the spatial distri-
bution of BVS values using structure data obtained from
combined neutron and X-ray diffraction data (Khasanova et
al., 2015) suggested one-dimensional Li-ion diffusion along
the [010] direction with de/intercalation of one Li ion per
formula unit from the ‘most open’ Li1 site, while participation
of the Li2 site located on the isosurface edge was found to be
questionable (Fig. 7a).
Investigation of the electrochemical properties of these
fluoride phosphates is hampered by their high redox poten-
tials: �5 V for Li2CoPO4F and even higher (>5.3 V) for
Li2NiPO4F (Okada et al., 2005; Nagahama et al., 2010). During
the first charge, Li2CoPO4F was found to undergo an irre-
versible structural transformation involving mutual rotations
of CoO4F2 octahedra and PO4 tetrahedral units with a 5%
unit-cell expansion (Khasanova et al., 2011). This expansion,
which makes the framework ‘more open’, is expected to
facilitate Li mobility upon subsequent cycling. This assump-
tion was confirmed by the calculation of the spatial distribu-
tion of BVS values carried out for the chemically oxidized
fluoride phosphate Li1.3CoPO4F [space group Pnma, with a =
10.9492 (9) A, b = 6.2831 (8) A, c = 11.680 (11) A and V =
760.43 (1) A3]. As can be seen from Fig. 7(b), this transfor-
mation resulted in considerable expansion of the diffusion
channels along the b axis, with involvement of the Li1 and Li2
sites in the diffusion process, which would supply a higher
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IUCrJ (2015). 2, 85–94 Evgeny V. Antipov et al. � New materials for Li-ion batteries 91
Figure 4The crystal structure of the Na2MnPO4F fluoride phosphate. The MnO4F2
octahedra are shown in blue, phosphate tetrahedra in green and F in grey.The inset indicates the alkali metal positions.
Figure 5BVS mapping of Li+-ion transport pathways in Li2MnPO4F: projectionsin the (101) and (011) layers. The inset indicates the alkali metal positions.
Figure 6The crystal structure of the Li2CoPO4F fluoride phosphate. The CoO4F2
octahedra are shown in blue, phosphate tetrahedra in green and F in grey.Alkali metal positions are indicated in the inset.
specific capacity. Furthermore, this transformation seemed to
provide an additional curved Li-migration pathway along the c
axis, which would improve Li-ion transport (Khasanova et al.,
2015).
According to the capacity–voltage dependence obtained by
potentiostatic step measurements, the extraction of more than
one Li+ ion from Li2CoPO4F should take place at potentials
>5.5 V, i.e. beyond the stability window of commercial elec-
trolytes. Li2CoPO4F delivers a reversible discharge capacity of
60 mA h g�1 when the potential window is limited to 5.0 V
versus Li+/Li0, and this value is increased up to 90 mA h g�1
when the upper potential limit is shifted to 5.5 V (Khasanova
et al., 2011). An initial discharge capacity of 130 mA h g�1
(corresponding to intercalation of �0.9 Li) was detected in a
high-voltage electrolyte with fluorinated alkyl carbonates, but
noticeable capacity fading was observed upon prolonged
cycling (Amaresh et al., 2012). Therefore, evaluation of the
electrochemical performance of Li2CoPO4F and its practical
potentials is restricted by the inaccessibility of stable high-
voltage electrolytes.
Another way of exploring this fluoride phosphate system is
to adjust (to decrease) the operating voltage of these
compounds to values sustained by conventional electrolytes.
This might be achieved through a complete or partial substi-
tution of Co2+ by transition metals with lower values of M 2+/
M3+ redox potentials (Fe2+ or Mn2+). The Li2Co1�xFexPO4F
and Li2Co1�xMnxPO4F systems were found to exhibit very
limited ranges of solid solution, with little effect on the redox
potential value (Khasanova et al., 2013). These results are
explained by the large difference in the ionic sizes of these
transition metals: the framework seems to become unstable
upon higher substitution of Co2+ (ionic radius 0.735 A) by
larger Fe2+ (0.780 A) and Mn2+ (0.820 A) [ionic radii from
Shannon (1976)]. Indeed, while Li2MPO4F (M = Co, Ni) can
be prepared by direct synthesis, the synthesis of three-
dimensional Li2FePO4F requires electrochemical ion
exchange using the Na counterpart, NaLiFePO4F, and the
corresponding Mn-based fluoride phosphate has not yet been
identified.
The NaLiFePO4F phase was obtained by solid-state synth-
esis, reacting an equimolar mixture of NaF and LiFePO4
(Khasanova et al., 2012; Ben Yahia et al. 2012). It was found
that, upon cycling in an Li cell, this compound undergoes a
compositional transformation, giving a rise to a new poly-
morph of Li2FePO4F with a three-dimensional structure. This
new phase exhibited a sloping voltage profile at an average
potential of 3.4 V, delivering a reversible capacity of
113 mA h g�1 (�0.84 Li) with a 1.7% volume change between
charged and discharged states (Khasanova et al., 2012). This
good cycling performance, the small change in unit-cell
volume and the solid-solution electrochemical behaviour,
which implies the absence of serious kinetic limitations, make
this fluoride phosphate system suitable for a reversible
de-/intercalation of Li, especially if the energy density of this
system can be enhanced.
4. Conclusions and future outlook
Mixed Li and transition metal fluoride phosphates exhibit a
rich crystal chemistry, owing to the variety of association
modes of MOxFy polyhedra (mainly MO4F2 octahedra) and
PO4 tetrahedra in polyhedral networks of different dimen-
sionality, thus providing a large playground for the design of
new cathode materials for Li-ion batteries. Several types of
such compounds, which are currently studied by different
research groups, and their relevant electrochemical properties
are listed in Table 1. This field of inorganic and materials
chemistry is obviously very interesting for the academic
community because it opens up great perspectives for the
discovery of new structure types and unconventional synthesis
routes. However, the practical application of these materials
raises serious doubts when compared with LiFePO4 due to the
following reasons:
(a) More complicated (and costly) synthesis techniques;
(b) Smaller (in general) gravimetric and volumetric energy
densities for cheap Fe-based compounds if only one Li ion is
extracted;
(c) Side-reactions with electrolyte and, consequently, a low
Coulomb efficiency for high-voltage Co-based materials;
(d) A relatively large volume variation for V-based fluoride
phosphates upon Li extraction or its dissolution in organic
electrolytes for the heavily reduced phase.
Nevertheless, these compounds exhibit several advanta-
geous properties that make them not simply ‘objects of
scientific curiosity’ but provide some hopes for practical
applications. The most important are the following:
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92 Evgeny V. Antipov et al. � New materials for Li-ion batteries IUCrJ (2015). 2, 85–94
Figure 7BVS maps of Li+-ion migration in (a) the Li2CoPO4F structure and (b)the chemically oxidized Li1.3CoPO4F phase. Projections in the (101) and(110) layers are given. The inset indicates the alkali metal positions.
(a) These phases may have a relatively high Li-ion diffusion
coefficient, due to the greater free volume for Li-ion migration
compared with close-packed mixed oxide structures, and
weaker Li+—F� bonding compared with Li+—O2� ones. This
feature is important for high-power batteries and their appli-
cation at low temperatures.
(b) Several phases have high values of specific energy,
significantly exceeding the value for LiFePO4. They can be
used for batteries with high gravimetric energy densities, if the
problem with high-voltage liquid electrolytes can be solved, or
for applications in all solid-state ceramic batteries.
(c) Obviously, the extraction of more than one Li ion per
formula will provide a significant increase in specific energy.
However, the problem of a stable high-voltage electrolyte
would be the ‘bottle-neck’, as in the previous case.
(d) Last, but not least, some of these phases are attractive
for application in Na-ion batteries because they can be easily
produced and Na ions exhibit high mobility, as shown by Li-
based analogues obtained by ion-exchange chemically or
electrochemically.
The rapid development of the rechargeable batteries
industry requires the exploration of different types of material
because their properties are the most important factor limiting
progress in this field. The LIB is a complicated system
combining different types of inorganic, organic and polymer
materials. Mixed polyanion-based fluorides may open up new
possibilities once the respective problems are solved for the
whole set of materials to be used in LIBs.
APPENDIX ABVS landscape computational procedure
The algorithm for calculating BVS mismatch maps is compe-
tently implemented in the 3DBVSMAPPER program (Sale &
Avdeev, 2012), which was utilized in this work. The program is
capable of automatically producing a three-dimensional
canvas of BV values for a given test ion (Li+), based on the
following equation
BVS ¼XN
j¼1
mj expR0 � dj
b
� �� �; ð2Þ
where dj is the distance to the jth counter-ion site with occu-
pancy mj, R0 and b are tabulated constants which are depen-
dant on the type of Li+ ion and the jth neighbour, respectively,
and N is the number of counter-ions within the present cut-off
distance.
This formula does not consider Coulomb repulsion, but
3DBVSMAPPER generates blocking spheres around atoms
with the same sign of oxidation state as Li+ (in this work, P5+
and Mn+) to avoid building unphysical landscapes. Also, the
initial position of Li+ ions is ignored when performing the BVS
calculation to analyse the crystal structure for all possible ionic
transport pathways.
Crystallographic information files (CIFs) for Li2FePO4F
and Li2MnPO4F were artificially created by substitution of
sodium by lithium in the original CIFs for the Na counterparts
(Na2FePO4F and Na2MnPO4F, respectively; Ellis et al., 2007;
Yakubovich et al., 1997) and shrinkage of the unit cell to the
values refined from X-ray powder diffraction patterns. All the
BVS landscapes within the scope of this article were calculated
according to a �0.1 deviation of BV value from the control
value for an Li+ ion (which is equal to 1) with a fine spatial
resolution of 0.2 A. The magnitude of the BVS deviation is
scaled in a reverse-rainbow manner, ranging from dark blue
(0.0) to red (0.1) in both directions.
Acknowledgements
This work was supported by Skolkovo Institute of Science and
Technology, Russian Foundation of Basic Research (grant No.
13-03-00495a) and the Lomonosov Moscow State University
Program of Development up to 2020.
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Table 1Selected electrochemical properties of Li and transition metal fluoride phosphates.
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